Telemetry system



July 28, 1964 c. M. JOHNSON 7 TELEMETRY SYSTEM Filed May 14, 1962 2 Sheets-Sheet l ENCODER SENSOR mvmoa CHARLES M. JOHNSON BY WM 72 4% AGENT July 28, 1964 c. M. JOHNSON 3,142,837

TELEMETRY SYSTEM Filed May 14, 1962 2 Sheets-Sheet 2 FIG.

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nited States atent 3,142,837 TELEMETRY SYSTEM Charles M. Johnson, Towson City, Md., assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed May 14, 1962, Ser. No. 194,256 3 Claims. (Cl. 343100) This invention relates to communications systems and more particularly to a telemetry communications system utilizing a modulated reflector.

Present communications systems for space telemetry suifer from a variety of difficulties. Among these are (1) low reliability due to the finite lifetime of the R-F transmitters used in the satellites and (2) the lack of communications privacy and susceptibility to jamming due to the fixed frequency operation. Provisions for narrow beam antennas on the satellite eliminate some of these problems, but add new ones in the form of antenna steering and vehicle stability requirements.

A known method of ground-based data communications encompasses the use of modulated reflectors. In these systems, an unmodulated carrier wave is directed at the reflector from a transmitter-receiver station. The carrier is then simultaneously modulated with information by the reflector and reflected back towards the transmitter-receiver station. In this manner, the requirement for an active transmitter at the reflector station is eliminated. Until recently, the most suitable type of reflector for this purpose was a modified corner reflector having a movable wall. By controlling the movements of the wall with electrical signals, an unmodulated carrier was simultaneously modulated and reflected. Of course, this system was not only bulky, but it also consumed high power and was frequency limited. These problems precluded the use of such a system in space satellites. For these reasons, only completely active satellites, e.g. having both receivers and transmitters, have been thought feasible for space telemetry purposes. These, of course, have the drawbacks of large power consumption and short lifetime.

Accordingly, it is an object of this invention to provide means for deriving data from a satellite which does not require an on-board active transmitter.

It is another object of this invention to provide an improved electromagnetic modulator-reflector.

It is a further object of this invention to provide a narrow beam elecromagnetic modulator-reflector suitable for use in space satellites.

7 Still another object of this invention is to provide a modulator-reflector which does not exhibit undesirable reflective characteristics under certain modes of modulation and thereby provides for privacy of communications.

A recent patent to L. C. Van Atta, 2,908,002, describes an electromagnetic reflector which has been found particularly suited to satellite applications. Describedtherein is a reflector which comprises an array of antennas symmetrically disposed about a geometrical center. Symmetrical pairs of antennas are interconnected by transmission lines of equal electrical length. Inasmuch as the delay provided to each portion of a received wave front by each transmission line is equal, the array reflects the received energy back in the direction from which it was received. The reflecting array disclosed by Van Atta is not only equally sensitive to signals received from over a wide angle of view, but also has the characteristic of forming the reflected energy into a very narrow beam width.

In accordance with the above stated objects, this invention comprises the use of the Van Atta array in combination with bilateral information modulation means inserted in each electrical transmission path between each pair of antennas. In this manner, received, unmodulated electromagnetic energy may be caused to be modulated as it is reflected back to a receiving station. The bilateral modulation means are randomly spaced within the electrical transmission path, to prevent the reflector-modulator from appearing as a flat plate when the electromagnetic energy is blocked by the modulating means.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

FIG. 1 is a schematic view of a ground-satellite configuration wherein the subject invention is employed.

FIG. 2 is a circuit block diagram of an embodiment of this invention.

FIG. 3 is a block diagram of a configuration of this invention wherein modulators are symmetrically disposed about the center line of the antenna array.

FIG. 4 is a series modulator suitable for use with the circuit shown in FIG. 2.

FIG. 5 is a shunt modulator usable with the circuit shown in FIG. 2.

As stated in the introduction and more specifically described in the aforementioned patent, the Van Atta reflecting array effects a narrow beam reflection of an incident electromagnetic wave back in the direction from which it was received. This is accomplished by subjecting all portions of the received Wavefront to an equal delay while inverting it and reversing its direction of propagation.

With reference to FIG. 1, satellite 10 is shown in an orbit about earth. The upper portion 12 of satellite 10 contains any desired data sensing instrumentation and the lower portion houses the reflector-modulator 14. On earth, transmitting antenna 16 transmits an unmodulated carrier 18 beamed at satellite 10. Within the reflectormodulator 14 in satellite 10, the unmodulated carrier is modulated by the information derived from the sensors and reflected back towards receiving antenna 20 in the form of a narrow modulated beam 22. Receiving antenna 20 and transmitting antenna 16 are located at substantially the same position on earth. This method of satellite-to-ground data transfer eliminates the requirements for a large power source within the satellite to drive an active transmitter. Additionally, the wide angle of view of the Van Atta reflector precludes the need for precise attitude control of satellite 10.

FIG. 2 is a block diagram of a reflector-modulator 14. Conjugate pairs of antennas 22-22, 24-24, and 26-26 are connected by electrical transmission lines 28, 30 and 32 respectively. All of the electrical transmission lines 28, 30 and 32 are substantially equal in electrical length and each thereby acts to impart an identical electrical delay to a received electromagnetic signal. While only 3 pairs of antennas are shown, in practice it is desirable to use as many pairs as possible since, by so doing, the gain of the array is enhanced and the reflected beam width is narrowed.

Antennas 22-22, 24-24 and 26-26 may be of any well known type, depending upon polarization and weight requirements. One antenna which is suitable for such a configuration is the printed spiral or helical antenna element. Electrical transmission lines 28, 30 and 32 may also be of any well known type, the choice being dependent upon the frequency of the received electromagnetic energy, weight, and tolerable attenuation values. A preferred electrical conductor which represents a reasonable compromise between all of the aforementioned factors is the well known flexible coaxial transmission line. This cable, besides being light in weight, has the virtue of being able to transmit wide frequency ranges of electromagnetic energy with little attenuation.

Inserted in each of transmission lines 28, 30 and 32 are modulators 34, 36 and 38 respectively. Of the various types of modulation devices which could be incorporated in these modulators, the microwave diode switch is preferred since 'it combines maximum reliability and simplicity with minimium'power requirements and weight. There are several diode switches available which perform equally well as modulators-the variable resistance diode or the variable reactance or capacitance diode. Depending uponthe R-F configuration chosen for the diode, isolation ranging from 8 to 30 db, insertion losses of less than 1 db and switching times as low as 20 nanoseconds are presently available. Additionally, the power required to modulate the diodes is quite minuteas low as nanowatts watts). for variable reactance diodes operated between two negative bias points.

The diode switches are inserted either as serieselements or shunt elements in each of the transmission lines 28, 30 and 32. A series connected diode modulator is shown in FIG. 4' and a shunt connected diode modulator is shown in FIG. 5 (described in greater detail herein after). Each diode modulator acts as a switch and, dependent upon the bias applied thereto, either closes or opens its respective transmission line to the passage of electromagnetic energy. In this manner, either pulse code or pulse duration modulation may be accomplished by selectively biasing the diode modulators'in accordance with the desired code.

A phase reversal scheme of modulation may also be employed. This would entail providing each transmission path with a parallel circuit whose length is at least a half wave length longer than the normal transmission path. By switching the paths in accordance with a binary code, a ONE could be represented by a phase reversed ZERO signal. Additional schemes of modulation (e.g. analog) are also adaptable to this apparatus.

Sensor 40 is a device capable of changing a physical manifestation into a proportional analog voltage. It may be the type used to sense cosmic rays, nuclear radiation, temperature, etc. The analog voltage output from sensor 40 is fed to encoder 42 where it is converted to a suitable digitally coded modulating voltage. An exemplary encoder may be'found at pp. 177-l88 in Aerospace Telemetry by H. L. Stiltz, published in 1961 by Prentice Hall. The coded output from encoder 42 is simultaneously applied via conductor 44 as a modulating bias to each of diode modulators 34, 36 and 38. As above mentioned, each diode modulator basically acts as a switch, in that, dependent upon its bias, it either opencircuits or short-circuits its respective transmission line.

If simple variable resistance series diode modulators (i.e. FIG. 4) are used and they are biased for conduction, they appear as short circuits and do not affect the propagation of the received carrier. The operation of the system under this circumstance can best be understood by assuming that incident wavefront 104 impinges upon the array with an angle of incidence 6. Letters A, B, C, D, E and F indicate the portions of incident wavefront 104 which are received by antennas 22, 24, 26, 26', 24' and 22' respectively. The letters encircled by dotted lines indicate the rearrangement of the reflected wavefront as it emerges and is reradiated from the antenna array. (The reflected wavefront 106 is pictured as coincidentally positioned with and therefore hidden by incident wavefront 104). Since the modulators 34, 36 and 38 are assumed to be fully conductive, a signal will propagate from one antenna to its conjugate antenna without interruption. Specifically, portion A of incident wavefront 104 will be received by antenna 22, propagate down transmission line 28, through modulator 34 and thence to antenna 22' where it will be reradiated. Likewise, portion F of incident wavefront 104 will traverse an identical path as that of portion A, but in the oppo site direction. Note, that when portion F'of incident tennas.

Wavefront 104 just emerges from antenna 22, portion A will be at a distance of 5b from antenna 22. Likewise, when it is remembered that each transmission line imparts an identical electrical delay to a received signal, it can be seen that when portion B has emerged to a distance of 411 from antenna 24', portion B will be located at a distance b from antenna 24. Each portion of the in cident wavefront is therefore delayed an equal amount; its position inverted with respect to other portions; and its direction of propagation reversed through the action of the antenna array. By virtue of this phenomenon, the relationships are re-established and reinforce one another along a line coincident with the received wavefront and the main reflection lobe is directed back in the direction from which the incident wave was received.

If now, the diode modulators are biased to their nonconductingstate, they appear as open circuits and prevent the passage of the carrier between conjugate an- It can be seen that if all of the diode modulators were symmetrically positioned about the center line of the modulator-reflector array and biased to their open circuit state, the array would coherently reflect the energy at an angle of reflection equal to the angle of incidence (similar to a flat plate). This phenomenon can best be understood by referring to FIG. 3 where a simplified version of the modulator-reflector array is shown. Here, insteadof the modulators being staggered as they are in the preferred embodiment of this invention, they are symmetrically placed about the center line of the antenna array. Assume that modulators 80 and 82 are biased so that they open circuit their respective transmission lines 84 and 86. If incident wavefront 88, having an angle of incidence 6 is received by antennas 72,

. 74, 76 and 78, and impressed upon the transmission lines,

the respective portions of wavefront 88 will be reflected by modulators 80 and 82 back towards the receiving antennas. More specifically, portion (A) of incident wavefront 83 will travel a distance from the time it is received at antenna 72 to the time it again passes back in the opposite direction through antenna 72. At the time portion (A) impinges upon antenna 72 portion (D) of incident wavefront 88 still has the distance 3b to travel before it even reaches antenna 78. Thus, when portion (D) of incident wavefront 88 has been received, reflected and is just emerging from antenna 78, portion (A), which now constitutes part of having an angle equal to the angle of incidence. Because the reflected wave contains the mirror image of the information being impressed upon the carrier and is dir'ected away from the receiving station, this configuration of the system does not have the desirable attribute of communications privacy. To overcome this problem, it has been found that when the modulators are randomly staggered about the center line of the modulator-reflector, the reflected energy is prevented from reconstituting itself when the diodes open circuit their respective transmission lines. By randomly staggering the modulators as shown in FIG. 2- a non-coherent phase addition occurs which causes a diffusion of the reflected energy and a resultant loss of the directional and long range propagation properties of the modulator-reflector.

In FIG. 4 there is shown a series fed diode modulator adapted for use in this invention. The modulating voltage 'is applied between the capacitor 50 and inductor 46 to diode 52. Inductor 48 provides a return path to ground. Inductors 46 and 48 also act as R-F isolation elements to prevent the R-F signal on the center conductor from being applied to the ground conductor. Capacitors 54 and 56 act as D.C. isolation elements. When a positive bias is applied to the anode of diode 52, it passes the R-F energy in an unimpeded form. When, however, a negative bias is applied to the anode of diode 52 it appears as an open circuit to the R-F energy and results in a reflection thereof.

FIG. 5 shows a shunt diode modulator. In this case, a positive bias applied to the anode of diode 56, causes it to appear to the R-F energy as a radical change in the transmission lines impedance. This results in a substantially complete reflection of the incident energy. When, however, there is a negative bias applied to the anode of diode 56, the diode appears as an open circuit and does not affect the propagation of the electromagnetic energy.

If variable capacitance diodes are employed in lieu of variable resistance diodes, much less switching power is required. These diodes, when combined with suitable parallel or series connected inductances provide a switching function by utilizing resonance characteristics. Assuming a parallel resonant network is used, it will appear as high impedance when the diode is biased so that the circuit resonates at the carrier frequency; whereas off resonance, the opposite will be true. In a series resonant circuit the reverse action is observed. Since the diode is back-biased to provide the variable capacitance, only reverse current is drawn (less than amperes) with a resultant negligible consumption of power.

To summarize, the operation of the total system is as follows:

(FIGS. 1 and 2). Transmitting antenna 16 transmits an unmodulated carrier 18 towards the satellite 10 wherein there is housed modulator-reflector 14. If at this time it is desired to transmit a binary ONE bit from modulator-reflector 14, each of modulators 34, 35 and 38 is biased to short circuit its respective transmission line. Thus, when the unmodulated carrier 18 impinges upon antennas 212-22, 24-24 and 26-26 its propagation along transmission lines 28, 30 and 32 is unimpeded. Under these circumstances, the reflected carrier energy is directed back towards receiver 20.

Now, if it is desired to transmit a binary ZERO from the satellite, all of diode modulators 34, 35 and 38 are biased so that they open circuit their respective transmission lines 28, 30 and 32. Thus, when unmodulated carrier 18 is received it is reflected and dispensed in such a manner that receiver 20 receives no return signal. By correlating the returned and non-returned signals, receiver 20 can thereby synthesize the digital code created in the satellite by encoder 22 and derive the desired information therefrom.

It should be realized that certain changes can readily be made in the system. For instance, instead of providing separate receiving and transmitting antennas at the ground station, a single antenna in combination with a frequency diversity transmitter and receiver could be employed. Thus, while the antenna was transmitting a first carrier frequency, the receiver would be tuned to receive a carrier frequency previously transmitted. The transmitter and receiver would be ganged together so that the receiver could always track the previously transmitted carrier signal.

When the above-described telemetry system is compared with a conventional active telemetry system from the standpoint of satellite power requirements, its true merit is realized. For example, it can be shown that for an orbit altitude of 1000 nautical miles and a required carrier signal band width of 10 mc., that an active satellite requires 200 times more on-board power than is required in the above-described system. If variable capacitance diodes are used in this system (requires only nanowatts to modulate) the active satellite requires almost 20,000 times as much power.

While the invention has been particularly shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made without departing from the spirit and scope of the invention.

I claim:

1. An electromagnetic modulator-reflector, comprising:

a plurality of pairs of antennas, the antennas of each said pairs being symmetrically disposed about a geometric center common to all said pairs;

an electrical transmission path connecting the antennas of each of said pairs, all said electrical transmission paths being substantially equal in electrical length: and

a plurality of modulation means, one in each said electrical transmission path, each said modulation means being randomly placed between its respective pair of antennas.

2. An electromagnetic modulator-reflector, comprising:

a plurality of pairs of antennas, the antennas of each said pairs being symmetrically disposed about a geometric center common to all said pairs;

an electrical transmission path connecting the antennas of each of said pairs, all said electrical transmission paths being substantially equal in electrical length; and

a pulse modulator in each said electrical transmission path, each said pulse modulator being randomly placed in its respective transmission path.

3. A communications system which includes a satellite,

a source of electromagnetic energy adapted to radiate said energy in the direction of said satellite, and a receiver positioned with said source, said satellite comprising:

a plurality of pairs of antennas, the antennas of each said pairs being symmetrically disposed about a geometric center common to all said pairs:

an electrical transmission path connecting the antennas of each of said pairs, all said electrical transmission paths being substantially equal in electrical length;

pulse modulation means in each said electrical transmission path, each said means being randomly placed between each said pair of antennas; and

sensor means connected to control said modulation means. 

1. AN ELECTROMAGNETIC MODULATOR-REFLECTOR, COMPRISING: A PLURALITY OF PAIRS OF ANTENNAS, THE ANTENNAS OF EACH SAID PAIRS BEING SYMMETRICALLY DISPOSED BOUT A GEOMETRIC CENTER COMMON TO ALL SAID PAIRS; AN ELECTRICAL TRANSMISSION PATH CONNECTING THE ANTENNAS OF EACH OF SAID PAIRS, ALL SAID ELECTRICAL TRANSMISSION PATHS BEING SUBSTANTIALLY EQUAL IN ELECTRICAL LENGTH. AND A PLURALITY OF MODULATION MEANS, ONE IN EACH ELECTRICAL TRANSMISSION PATH, EACH SAID MODULATION MEANS BEING RANDOMLY PLACED BETWEEN ITS RESPECTIVE PAIR OF ANTENNAS. 